gnu: mercurial: Patch to make it read HGEXTENSIONPATH.

This will make Mercurial be able to find third-party extensions installed with
Guix, without having to set PYTHONPATH.

* gnu/packages/patches/mercurial-hg-extension-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register the patch.
* gnu/packages/version-control.scm (mercurial)[origin](patches): Apply the
patch.
[native-search-paths]: Add HGEXTENSIONPATH.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/local.mk

@@ -1413,6 +1413,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/mcrypt-CVE-2012-4527.patch			\
   %D%/packages/patches/libmemcached-build-with-gcc7.patch	\
   %D%/packages/patches/libmhash-hmac-fix-uaf.patch		\
+  %D%/packages/patches/mercurial-hg-extension-path.patch       \
   %D%/packages/patches/mesa-skip-tests.patch			\
   %D%/packages/patches/mescc-tools-boot.patch			\
   %D%/packages/patches/meson-for-build-rpath.patch		\

gnu/packages/patches/mercurial-hg-extension-path.patch

@@ -0,0 +1,29 @@
+This is needed to make Mercurial read the HGEXTENSIONPATH to detect
+third-party extensions.  It is called HGEXTENSIONPATH and not
+HG_EXTENSION_PATH to keep it consistent with other environment variables for
+Mercurial, e.g. HGENCODINGAMBIGUOUS, HGEDITOR ...  Hopefully I or someone else
+will get this into Mercurial proper.
+
+diff --git a/mercurial/extensions.py b/mercurial/extensions.py
+--- a/mercurial/extensions.py
++++ b/mercurial/extensions.py
+@@ -13,6 +13,7 @@
+ import imp
+ import inspect
+ import os
++import sys
+ 
+ from .i18n import (
+     _,
+@@ -108,6 +109,11 @@
+ 
+ def _importh(name):
+     """import and return the <name> module"""
++    # Read HGEXTENSIONSPATH environment variable when import extensions.
++    extension_path = os.getenv("HGEXTENSIONSPATH")
++    if extension_path is not None:
++        for path in extension_path:
++            sys.path.append(path)
+     mod = __import__(pycompat.sysstr(name))
+     components = name.split(b'.')
+     for comp in components[1:]:

gnu/packages/version-control.scm

@@ -1614,6 +1614,7 @@ (define-public mercurial
              (method url-fetch)
              (uri (string-append "https://www.mercurial-scm.org/"
                                  "release/mercurial-" version ".tar.gz"))
+             (patches (search-patches "mercurial-hg-extension-path.patch"))
              (sha256
               (base32
                "17rhlmmkqz5ll3k68jfzpcifg3nndbcbc2nx7kw8xn3qcj7nlpgw"))))
@@ -1684,6 +1685,11 @@ (define-public mercurial
        ("which" ,which)))
     (inputs
      `(("python" ,python)))
+    ;; Find third-party extensions.
+    (native-search-paths
+     (list (search-path-specification
+            (variable "HGEXTENSIONPATH")
+            (files '("lib/python3.8/site-packages/hgext3rd")))))
     (home-page "https://www.mercurial-scm.org/")
     (synopsis "Decentralized version control system")
     (description